I think Chuck is about 13 years older than Jimmy.
Let's look at Season 2 Episode 5, when Chuck has his heart-to-heart with Kim about Jimmy and basically makes his case for why Jimmy is a scumbag.
Chuck tells Kim he was at college when Jimmy first went to work for his dad at the store. He also says he came back home from a clerkship to help his dad with the business and discovered that over the years Jimmy had stolen $14k from the till in "dribs and drabs." He couldn't save the business and his dad died 6 months after it closed.
Now let's look at Season 2 Episode 7, where we see Jimmy at around 13 years old working at the store with his dad. We don't know if Chuck is home yet or if Jimmy has been working for 5 years or 5 days.
We can make a reasonable assumption that when Chuck comes home he is likely around 26 or 27. Why? Because clerkships happen either as a 3L (third year law student) or immediately following law school and last for no more than a few years. Most last a year or two. College at 18, law school at 22, clerkship at 24 or 25. We can be generous and say Chuck was nearing the end of his clerkship (hence why he was ok with leaving it) and so was 27.
Let's say Chuck was home and already trying to save the business when we see young Jimmy in episode 7. That would put Jimmy at 13 and Chuck at 27. 14 years apart, max.
But what if that was the first time Jimmy stole from the till? What if he still needed years to accumulate the $14k in dribs and drabs? That scene was obviously meant as a formative moment in Jimmy's life, when he sees the difference between wolves and sheep. It's very reasonable to assume he made his choice then and there, that it was the first time he took advantage of his dad and stole from him. That would mean he was years older when Chuck came home, putting Jimmy at more likely around 17 or 18 (or even older), with Chuck at 27. That puts them 10 years apart, max.
I think the most likely scenario is episode 7 was the first time Jimmy stole from his dad, over the next 3 or 4 years he took the $14K, and was 17 when Chuck came home at 27.
He's 10 years younger than Chuck.